Lubricating compositions are used to prevent damage to machinery under operating conditions. Typically, modern lubricants contain additives that perform a variety of protective functions. Example additives include antioxidants, extreme pressure agents and anti-wear agents. Dithiophosphate derivatives in particular have been used as anti-wear agents to protect metal parts under boundary lubricating conditions thereby minimizing harmful metal-to-metal contact.
In practice, lubricants are frequently transported long distances over a period of days, sometimes weeks. Furthermore, lubricant products may sit in various storage facilities prior to being made available for purchase to the end user. Due to the complex chemical nature of lubricant compositions certain in situ reactions may take place during the transport or storage period of a lubricant. Lubricant compositions utilizing dithiophosphate components in particular are susceptible to in situ reactions thereby creating “off spec” lubricants, that is, lubricant compositions that do not meet the manufacturer's lubricant performance specifications. Using “off spec” lubricants creates uncertainty in the effectiveness of a lubricant and in some cases actual damage may occur to the machines in which these lubricants are used. For example, anti-wear components that decompose in situ may lose their effectiveness and increase the wear of metal parts. Accordingly, there has been a long felt need to have more stable dithiophosphate anti-wear chemistry, and in particular dithiophosphate chemistry that is resistant to in situ degradation while maintaining the required level of anti-wear performance.
A class of dithiophosphate anti-wear compounds was previously disclosed in U.S. Pat. No. 5,922,657. Other dithiophosphate anti-wear compounds have been described in U.S. Pat. No. 5,362,419. Finally, some lubricating properties of O,O-dialkyl-phosphorodithioates have been described by Huanmou et al. (Journal of Hunan University, vol. 23, no. 3, 1996, pp. 65-70). However, none of these anti-wear agents addresses the stability issues previously identified for this class of compounds.
It has now been discovered that certain novel compounds as described hereinbelow, having unique alkyl substitution on the dithiophosphate, have highly desirable stability properties, making the compounds resistant to in situ degradation, and may be readily formulated into lubricating compositions to provide excellent wear performance.